A number of technologies have been developed to repair corroded or damaged thin-walled, small diameter tubes used in applications such as heat exchangers or material transport systems. U.S. Pat. Nos. 5,430,270; 5,514,849; 5,430,270; 5,656,185; 5,573,683; and 5,653,897 disclose technologies of this type. Each of these patents is owned by the assignee of the present invention and is incorporated by reference herein. FIG. 1 illustrates an apparatus described in several of the foregoing patents. In particular, the figure illustrates a rotating apparatus 20 used to repair damaged tubes. A rotating welding head 22 is fixedly positioned at the end of a rotating sleeve 24.
A rotating drive mechanism 25 rotates the sleeve 24, thus the rotating sleeve 24 and the rotating welding head 22 synchronously rotate. The rotating drive mechanism 25 simultaneously rotates a filler assembly 26 that includes a filler metal receptacle 28 and a filler metal delivery system 30. The filler metal receptacle 28 holds the filler metal to be welded. Generally, the filler metal receptacle 28 will be in the form of a reel of filler metal wire. The filler metal delivery system 30 receives the filler metal and delivers it to a filler passage within the rotating sleeve 24. Since the rotating sleeve 24 and the filler assembly 26 rotate synchronously, the filler metal does not become tangled.
The filler metal delivery system 30 is powered through filler assembly slip rings 32. The speed of the wire feed motor can be varied to permit different wire feed speeds, providing control of clad thickness and to allow adjustment for variations in laser output levels, travel speed, rotational pitch, and other factors.
The rotating apparatus 20 also includes a gas coupler 36 that is connected to a gas supply 38. The rotating sleeve 24 includes a rotating fiber optic cable 40. A laser 44 supplies energy to a fixed fiber optic cable 43. The laser energy is transferred from the fixed fiber optic cable 43 to the rotating fiber optic cable 40 through an optical coupler 42.
The rotating apparatus 20 is moved along its longitudinal axis by an axial drive system 50 mounted on shaft 51. Guide rollers 49 may be used to guide the rotating sleeve 24 into position. A computer controller 53 is used to control the operation of the rotating apparatus drive mechanism 25, the axial drive system 50, and the filler metal delivery system 30. In particular, the computer controller 53 is used to set the speed of the rotating apparatus drive mechanism 25, the position for the axial drive system 50, and the filler delivery rate for the filler metal delivery system 30.
The operation of the rotating apparatus 20 is more fully appreciated with reference to FIG. 2, which provides an enlarged cross-sectional view of the rotating welding head 22. The rotating welding head 22 includes a body 80, which defines a filler passage 86. The filler passage 86, also called the wire conduit runs the length of the rotating sleeve 24. Filler 88 is forced from the filler metal delivery system 30 through the filler passage 86 to a body aperture 94. The laser energy is delivered through the body aperture 94 and welds the filler 88. Gas conduit 89 delivers a shielding gas to the welding head 22. Preferably, the gas conduit 89 terminates in distribution channels that distribute the gas to the aperture 94 at a number of locations.
FIG. 2 also depicts the rotating fiber optic cable 40 positioned within the body 80 of the rotating welding head 22. The rotating fiber optic cable 40 runs the length of the rotating sleeve 24 and is affixed thereto.
The rotating fiber optic cable 40 terminates at a laser energy directional modification assembly 92. Preferably, the assembly 92 is implemented as an optical assembly. FIG. 3 discloses an assembly 92 that includes an input lens assembly 96, a wedge prism 97, and an output lens assembly 98. The wedge prism 97 serves to change the direction of the laser energy. Preferably, the laser energy is directed toward the receiving surface 99 at a non-orthogonal angle .theta.. When the laser energy is impinged upon a surface to be welded at an angle, of say 45.degree., as shown in FIG. 3, then reflective laser energy does not disrupt the incoming laser energy.
The device of FIGS. 1-3 has been used for clad weld repair of thin-walled (e.g., 0.05 inches thick) heat exchanger tubes. The device can also be used for fusing defects by melting and re-solidifying the metal of a thin-walled heat exchanger tube.
Most corrosion in pressurized water reactors has been associated with thin-walled heat exchanger tubes. However, there have been recent reports of water stress corrosion cracking in reactor pressure vessel control rod drive mechanisms. FIG. 4 illustrates a prior art reactor vessel dome 110 with a set of control rod drive mechanism (CRDM) nozzles 112. A prior art repair system is positioned underneath the reactor vessel dome 110. The prior art repair system includes a tool delivery system 114, which supports a tool arm 116 that has a tool head 118 positioned at its end. The tool delivery system 114 executes radial motion as shown with line 120, rotational motion as shown with arc 122, and lift motion as shown with line 124. These motions are used to deliver the tool head 118 to different locations in a CRDM nozzle 112 so that repairs can be effectuated.
A variety of tool heads 118 are used to effectuate repairs. A detection probe that uses eddy current techniques may be used to identify flaws in the CRDM nozzle 112. Similarly, a detection probe that uses ultrasonic testing may be used to identify flaws in the CRDM nozzle 112. A detection probe to execute dye penetrant examinations may also be used. Such a probe is used to verify information found from other detection techniques and to examine completed weld repairs.
An excavation tool may also be used as a tool head 118. Prior art excavation tools generally rely upon milling, grinding, or cutting tools. Such tools typically require large motor power that is difficult to deliver to remote locations, such as CRDM nozzles. Another class of prior art excavation tools relies upon a welding mechanism to melt damaged surface areas. The problem with this approach is that it is rather difficult to handle the molten metal that is removed from the damaged surface areas. Both of the foregoing excavation techniques also share the shortcoming that they are imprecise and therefore result in relatively large and unnecessary excavations that must be reconstructed.
A cavity repair weld head may be used for reconstruction operations. Such a weld head is used to fill the excavated area with a filler material, such as weld beads. Alternately, an arc welding cavity repair weld head may be used. For example, a gas-tungsten arc welding tool may be used.
A boring tool head may also be used as a tool head 118. A boring tool is used to bore the weld buildup after a weld repair. This allows the nozzle 112 to be returned to original design specifications.
As indicated above, one problem with prior art excavation tools is that they are imprecise and therefore produce relatively large excavations. Consequently, relatively voluminous reconstruction operations must be performed. This can result in high residual stresses and welding distortion, which may promote future cracks. Another problem arises when welding excavation operations produce a molten metal byproduct that is difficult to dispose. Finally, prior art techniques require a relatively large number of tool heads. It would be desirable to reduce the number of tool heads required to effectuate a repair.
In view of the foregoing, it would be highly desirable to provide an improved technique for repairing thick-walled components susceptible to corrosion, such as reactor pressure vessel control rod drive mechanisms in pressurized water reactor nuclear power plants. Such a technique should provide precision excavations to reduce the amount of reconstruction required. Further, such a technique should provide precision reconstruction welding operations to reduce residual stresses and welding distortion. Ideally, the technique would not produce a molten byproduct and would reduce the number of tool heads required to effectuate a repair.